The role of asexual reproduction in the production of pathogenic and genotypic variation in Aphanomyces euteiches was investigated. Variation was studied among three groups of 18 single-zoospore progeny of A. euteiches derived from each of three single-zoospore parental strains. Pathogenicity was assessed by evaluating disease severity (DS) on roots of five pea lines possessing different levels of resistance to Aphanomyces root rot and of a susceptible cultivar of snap bean and alfalfa. None of the single-zoospore progeny incited significantly higher DS levels than their parental strain on any of the seven hosts; however, 3 or 4 of the 18 progeny in each group incited significantly lower DS than their parental strains. The host range of the progeny either decreased or remained the same as compared with parental strains. Genotypic variation was assessed with randomly amplified polymorphic DNA (RAPD) analysis. Polymorphic RAPD markers that distinguished parental and progeny strains were detected within two of the three groups of strains with two of four RAPD primers used. Of 76 total RAPD markers that were detected among all strains in all groups, four (5%) were polymorphic. The polymorphic markers were not associated with the pathogenic variation.